The authors set up a simple and fairly rapid radioimmunoassay for vincristine, that shows a good sensitivity, precision and specificity. In particular, the higher specificity in comparison with other similar dosage techniques is likely due to the marked specificity of the used anti-vincristine serum, which displays a low interference even by molecules with a vincristine-like structure. Therefore, the suggested radioimmunoassay technique seems quite suitable for studying vincristine pharmacokinetics and for monitoring blood levels of this alkaloid in treated patients.